Safe mode and name node startup procedures

Details

Description

This is a proposal to improve DFS cluster startup process.
The data node startup procedures were described and implemented in HADOOP-124.
I'm trying to extend them to the name node here.
The main idea is to introduce safe mode, which can be entered manually for administration
purposes, or automatically when a configurable threshold of active data nodes is breached,
or at startup when the node stays in safe mode until the minimal limit of active
nodes is reached.

This are high level requirements intended to improve the name node and cluster reliability.
= The name node safe mode means that the name node is not changing the state of the
file system. Meta data is read-only, and block replication / removal is not taking place.
= In safe mode the name node accepts data node registrations and
processes their block reports.
= The name node always starts in safe mode and stays safe until the majority
(a configurable parameter: safemode.threshold) of data nodes (or blocks?)
is reported.
= The name node can also fall into safe mode when the number of non-active
(heartbeats stopped coming in) data nodes becomes critical.
= The startup "silent period", when the name node is in safe mode and is
not issuing any block requests to the data nodes, is initially set to a
configurable value safemode.timeout.increment. By the end of the timeout
the name node checks the safemode.threshold and decides whether to switch
to the normal mode or to stay in safe. If the normal mode criteria is not
met, then the silent period is extended by incrementing the safemode timeout.
= The name node stays in safe mode not longer than a configurable value of
safemode.timeout.max, in which case it logs missing data nodes and shuts
itself down.
= When the name node switches to normal mode it checks whether all required
data nodes have actually registered, based on the list of active data storages
from the last session. Then it logs missing nodes, if any, and starts
replicating and/or deleting blocks as required.
= A historical list of data storages (nodes) ever registered with the cluster is
persistently stored in the image and log files. The list is used in two ways:
a) at startup to verify whether all nodes have registered, and to report
missing nodes;
b) at runtime if a data node registers with a new storage id the
name node verifies that no new blocks are reported from that storage,
which would prevent us from accidentally connecting data nodes from a
different cluster.
= The name node should have an option to run in safe mode. Starting with
that option would mean it never leaves safe mode.
This is useful for testing the cluster.
= Data nodes that can not connect to the name node for a long time (configurable)
should shut down themselves.

Activity

This patch implements a part of the laid out design.
It stores the historical list of datanodes in the image file and logs newly
registered nodes in the edits file.
The changes are mostly related to the FSNamesystem class.
Since datanodes are not removed from the datanodeMap when they are considered
non responsive, the deadDatanodeMap field becomes redundant. I removed it.
There is a change in semantics of the relation between heartbeats and datanodeMap maps.
heartbeats contains only live nodes while datanodeMap contains both alive and dead nodes.
See JavaDoc for more details.
So when we are looking for new targets for block replication we should check the heartbeats
map rather than the datanodeMap as we did before.
Also since the DatanodeDescriptors are not physically removed from datanodeMap
I had to add their blocks cleanup while processing lost heartbeats.
Some changes to the FSImage and FSEditLog classes. I removed unnecessary
parameter to FSDirectory from the previous version. The whole name space can be
accessed via static method FSNamesystem.getFSNamesystem()

Konstantin Shvachko
added a comment - 11/Aug/06 21:54 This patch implements a part of the laid out design.
It stores the historical list of datanodes in the image file and logs newly
registered nodes in the edits file.
The changes are mostly related to the FSNamesystem class.
Since datanodes are not removed from the datanodeMap when they are considered
non responsive, the deadDatanodeMap field becomes redundant. I removed it.
There is a change in semantics of the relation between heartbeats and datanodeMap maps.
heartbeats contains only live nodes while datanodeMap contains both alive and dead nodes.
See JavaDoc for more details.
So when we are looking for new targets for block replication we should check the heartbeats
map rather than the datanodeMap as we did before.
Also since the DatanodeDescriptors are not physically removed from datanodeMap
I had to add their blocks cleanup while processing lost heartbeats.
Some changes to the FSImage and FSEditLog classes. I removed unnecessary
parameter to FSDirectory from the previous version. The whole name space can be
accessed via static method FSNamesystem.getFSNamesystem()

Doug Cutting
added a comment - 15/Aug/06 21:19 Konstantin, since this patch only partially fixes this issue, can you please instead add this patch to a separate issue that this issue depends on? Thanks!

I'd like to discuss the meaning of the safemode.threshold parameter, which defines
when the name node can leave safe mode and start replication/deletion of blocks.
I see at least 2 meanings:
1) % of storage ids reported so far. Storage id is a unique id of a data node
that identifies the storage rather than the data node address.
2) % of blocks reported so far by all data nodes.
They seem to be pretty independent. Even if we have 100% storage ids
reported that does not mean we have 100% blocks.
What 100% means for blocks? Does it mean the name node has
2a) at least one copy of each block or
2b) the complete list of replicas for each block.
All three constraints seem reasonable.
100% in (1) is the best one could physically do, since all storages are
reported, it is just that they have corrupted or missing blocks.
100% in (2a) is the minimal requirement for the name node to start
without any data loss, and
100% in (2b) would be the perfect state of the system.
So the question is what do we really want?

Konstantin Shvachko
added a comment - 22/Aug/06 20:19 I'd like to discuss the meaning of the safemode.threshold parameter, which defines
when the name node can leave safe mode and start replication/deletion of blocks.
I see at least 2 meanings:
1) % of storage ids reported so far. Storage id is a unique id of a data node
that identifies the storage rather than the data node address.
2) % of blocks reported so far by all data nodes.
They seem to be pretty independent. Even if we have 100% storage ids
reported that does not mean we have 100% blocks.
What 100% means for blocks? Does it mean the name node has
2a) at least one copy of each block or
2b) the complete list of replicas for each block.
All three constraints seem reasonable.
100% in (1) is the best one could physically do, since all storages are
reported, it is just that they have corrupted or missing blocks.
100% in (2a) is the minimal requirement for the name node to start
without any data loss, and
100% in (2b) would be the perfect state of the system.
So the question is what do we really want?

OTOH, I'd want the namenode to be able to move out of safe mode with minimal restrictions, to avoid unnecessary manual intervention.
OTOH, I want to avoid unnecessary thrash on startup, while data nodes are connecting.
So any solution would require both a threshold, and some timer once that threshold is reached, to avoid unnecessary replications the second the threshold is reached, before the last datanodes connect.

I'd go with a configuration requiring:

100% blocks are available, at least one replica

allow one more minute for stragglers to connect and report their blocks

typically this would allow up to two nodes to be missing, but if by some magic more nodes are missing and I still have all my data intact, such as when we implement rack aware data placement and a rack goes down, hallelujah - I wait one minute and start re-replicating.

Yoram Arnon
added a comment - 22/Aug/06 22:23 OTOH, I'd want the namenode to be able to move out of safe mode with minimal restrictions, to avoid unnecessary manual intervention.
OTOH, I want to avoid unnecessary thrash on startup, while data nodes are connecting.
So any solution would require both a threshold, and some timer once that threshold is reached, to avoid unnecessary replications the second the threshold is reached, before the last datanodes connect.
I'd go with a configuration requiring:
100% blocks are available, at least one replica
allow one more minute for stragglers to connect and report their blocks
typically this would allow up to two nodes to be missing, but if by some magic more nodes are missing and I still have all my data intact, such as when we implement rack aware data placement and a rack goes down, hallelujah - I wait one minute and start re-replicating.

I wonder if it makes sense to wait for 100% of blocks to be available. It's possible for data to go missing because of failed drives, nodes, racks... When some data goes missing it's usually the case that some other (co-located) data becomes under-replicated and the Namenode ought to start replicating the under replicated data. Why do we want safe mode?

To prevent replication thrash when the Namenode starts

To enable administrators to make the file system read only for diagnosis and debugging

Neither of these require that 100% of blocks are present. Maybe we should have a slightly lower threshold for blocks or storage ids.

Sameer Paranjpye
added a comment - 24/Aug/06 14:19 I wonder if it makes sense to wait for 100% of blocks to be available. It's possible for data to go missing because of failed drives, nodes, racks... When some data goes missing it's usually the case that some other (co-located) data becomes under-replicated and the Namenode ought to start replicating the under replicated data. Why do we want safe mode?
To prevent replication thrash when the Namenode starts
To enable administrators to make the file system read only for diagnosis and debugging
Neither of these require that 100% of blocks are present. Maybe we should have a slightly lower threshold for blocks or storage ids.

Thanks for the comments. Based on what you say I can define the threshold constraint as:

the percentage of blocks that meet minimal replication requirement (defined by dfs.safemode.replication)
I think this would be flexible enough.
And I'll keep the dfs.datanode.startupMsec that defines the startup period during which replications are not allowed.
So the name node will stay in safe mode at start up until the threshold is reached AND the startup period hasn't expired.

Konstantin Shvachko
added a comment - 24/Aug/06 21:50 Thanks for the comments. Based on what you say I can define the threshold constraint as:
the percentage of blocks that meet minimal replication requirement (defined by dfs.safemode.replication)
I think this would be flexible enough.
And I'll keep the dfs.datanode.startupMsec that defines the startup period during which replications are not allowed.
So the name node will stay in safe mode at start up until the threshold is reached AND the startup period hasn't expired.

the timeout should start after the threshold is reached.
If namenode is started long before datanodes, then as soon as the threshold is reached superfluous replications will occur.
the timeout should be short, I'd say less than a minute, and should start as soon as the data/nodes threshold is reached, to allow stragglers to connect and report

Yoram Arnon
added a comment - 25/Aug/06 19:22 the timeout should start after the threshold is reached.
If namenode is started long before datanodes, then as soon as the threshold is reached superfluous replications will occur.
the timeout should be short, I'd say less than a minute, and should start as soon as the data/nodes threshold is reached, to allow stragglers to connect and report

This is the safe mode implementation, which implements everything that was discussed here
except for the automatic entering safe mode when the number of non-active blocks falls below
the threshold. This seems to be arguable that rather than replicating as quickly as it can the
name node falls into safe mode.
Please see JavaDoc for complete descriptions of the safe mode concept and its implementation.

Other minor changes

Block report interval is configurable, but there was no value for it in hadoop-default.xml. I added it.
<name>dfs.blockreport.intervalMsec</name>

A typo in hadoop-default.xml

There was a memory leak related to that name node never removed blocks from the bloksMap
so it could grow large if the system works long enough. I fixed this unreported bug.

I moved the startup of http servers for both data and name nodes to the end of their constructors
as discussed in HADOOP-430

Konstantin Shvachko
added a comment - 14/Sep/06 23:26 This is the safe mode implementation, which implements everything that was discussed here
except for the automatic entering safe mode when the number of non-active blocks falls below
the threshold. This seems to be arguable that rather than replicating as quickly as it can the
name node falls into safe mode.
Please see JavaDoc for complete descriptions of the safe mode concept and its implementation.
Other minor changes
Block report interval is configurable, but there was no value for it in hadoop-default.xml. I added it.
<name>dfs.blockreport.intervalMsec</name>
A typo in hadoop-default.xml
There was a memory leak related to that name node never removed blocks from the bloksMap
so it could grow large if the system works long enough. I fixed this unreported bug.
I moved the startup of http servers for both data and name nodes to the end of their constructors
as discussed in HADOOP-430
ClientProtocol version is changed since setSafeMode() method is added

Doug Cutting
added a comment - 18/Sep/06 22:01 The enum mode should not be passed & returned as an int, but rather as a boolean or enum.
As a boolean this might look like:
boolean setMode(boolean isSafe);
boolean getMode();
To pass an enum over an RPC would require either changes to ObjectWritable or to make the enum class implement Writable.

Making enum implement Writable does not work, since you cannot implement
readFields() so that it'd modify fields (which are all final) in enum and change its state.

I made a simple modification to ObjectWritable that treats enumerator type as a s
pecial case like String or Array. This works fine for all simple (traditional) enum types.

If we will ever want to use internal fields and other "fancy" enum features, which are
considered to be confusing for most programmers anyway , then we will need to
introduce parameters to the WritableFactory.newInstance() that could be used to
construct a correct instance of the enum.

This patch in addition to what it had before introduces an enum type SafeModeAction
and a new functionality that lets RPC pass simple enumerator types.

Konstantin Shvachko
added a comment - 20/Sep/06 07:25 Making enum implement Writable does not work, since you cannot implement
readFields() so that it'd modify fields (which are all final) in enum and change its state.
I made a simple modification to ObjectWritable that treats enumerator type as a s
pecial case like String or Array. This works fine for all simple (traditional) enum types.
If we will ever want to use internal fields and other "fancy" enum features, which are
considered to be confusing for most programmers anyway , then we will need to
introduce parameters to the WritableFactory.newInstance() that could be used to
construct a correct instance of the enum.
This patch in addition to what it had before introduces an enum type SafeModeAction
and a new functionality that lets RPC pass simple enumerator types.